GRE tunnels not being created [closed]

I am setting up an OpenStack Havana deployment using 1 controller node, 1 compute node and 1 network node, with Neutron. I have chosen to use GRE tunneling as the networking technology.

However, the GRE tunnels necessary for virtual networking between the compute and network nodes are not being created due to the following error (which can be see in both network and compute hosts' /var/log/neutron/openvswitch-agent.log):